Question

The pH of a solution is $2$. Its pH is to be changed to $4$. Then the $H^{+}$ ion concentration of original solution has to be:

• A. Halved
• B. Doubled
• C. Increased $100$ times
• D. Decreased $100$ times

Answer 1

The correct option is D Decreased $100$ times

Formula for pH is $pH=-log\left[H^{+}\right]$

When $pH=2,$

$\left[H^{+}\right]={10}^{-pH}={10}^{-2}$

The pH of solution is increased to 4. So

When $pH=4,$

$\left[H^{+}\right]={10}^{-pH}={10}^{-4}$

The change in concentration of $\left[H^{+}\right]$ is ${10}^{-2}/{10}^{-4}={10}^{-2}=0.01$

It is decreased by 100 times.